This invention is related generally to electrodes for monitoring cortical electrical activity in order to define epileptogenic foci. However, the invention is not limited to monitoring brain electrical activity but also has improved features for monitoring and electrical stimulation of other tissue. For example, the invention may be used to monitor electrical activity in or to stimulate the spinal cord.
Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various methods have been used in attempting to determine epileptogenic foci, and all, of course, involve sensing of cortical electrical activity using electrical contacts applied in various ways.
Standard scalp contacts have been used for many years, but accurate localization is usually very difficult with recordings obtained from such contacts. Therefore, many epilepsy centers in recent years have used intracranial recording techniques to better define regions of cortical epileptogenicity.
Intracranial recording techniques have used either of two different types of electrodes--intracortical depth electrodes or subdural strip electrodes. The far more commonly used technique of intracranial recording uses intracortical depth electrodes, but other techniques using subdural strip electrodes, first utilized many years ago, have been shown to be relatively safe and valuable alternatives.
The relative safety of subdural strip electrodes lies in the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures with contact bands spaced along their lengths. Such electrodes are inserted into the brain in order to establish good electrical contact with different portions of the brain. Subdural strip electrodes, on the other hand, are generally flat strips supporting contacts spaced along their lengths. Such strip electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain.
A typical subdural strip electrode of the prior art is shown in FIGS. 1-4 and is disclosed in U.S. Pat. No. 4,735,208. The '208 patent discloses a subdural strip electrode 10 having an elongated flexible silicone dielectric strip 14, a plurality of spaced aligned flat electrical stainless steel contact disks 16 held within dielectric strip 14, and lead wires 18 exiting strip 14 from a proximal end 20 thereof.
Dielectric strip 14 of strip electrode 10 has front and back dielectric layers 22 and 24, respectively. Each front layer 22 has a front layer opening 26 for each contact disk 16. Openings 26 are circular and somewhat smaller in diameter than contact disks 16. Front and back layers 22 and 24 are sealed together by adhesive and/or heat such that they form, in essence, an integral dielectric strip.
As can be seen in FIGS. 2 and 3, the subdural strip electrodes of the prior art are predominately rectangular in cross-section. Other subdural strip electrodes of the prior art have a circular or round cross section.
As can be seen in FIGS. 5 and 6, prior art subdural strip electrodes that have a rectangular (FIG. 5) or round (FIG. 6) cross-section do not optimize the amount of surface area of the electrode E in contact with the cortical surface S. As the electrode E is inserted between the dura D and the cortical surface S, downward pressure is exerted by the dura on the electrode, which in turn exerts pressure on the cortical surface. Such pressure causes the cortical surface S to slightly deflect downward, as shown in the Figures. Prior art rectangular or round electrodes cannot follow this deflection, resulting, in the case of a rectangular electrode (FIG. 5), in the electrode contacting the surface S primarily at the edges; and in the case of the round electrode (FIG. 6), in the electrode contacting the surface S along an arc.
One of the problems with such prior art strip electrodes is that the lack of adequate contact with the brain surface can result in inaccurate recordings of epileptogenic foci.
Another problem with such prior art strip electrodes is that there is very little room between the dielectric strips to pass the wires. This may require very fine wires with high electrical resistance, which may in turn cause inaccurate recordings.
Another problem of such prior art strip electrodes is lack of adequate stiffness for insertion. It would seem that proper insertion of the strip electrode requires at least some degree of stiffness (less than complete flexibility) in the strip, because of how such strips are inserted through the burr hole and under the dura. That is, rather than being pulled into place between the dura and the brain by grasping the distal end of the strip, such strips must be pushed into place from their proximal ends from which the lead wires extend. Nothing supports the strip along its length much beyond the edge of the burr hole during the complete insertion step. It is understood that if there is insufficient stiffness along the strip length and across the strip width, the strip could not be inserted properly. In some cases, it could stray from the intended position; in other cases, it could turn or double up.
Proper insertion and positioning and having an accurate understanding of the exact positions of the strip are essential to proper interpretation of the recordings taken from the strip contacts. For that reason, it has been thought necessary to have a certain amount of strip thickness and width in order to provide the necessary body or stiffness for proper insertion.
Another consideration in the design of subdural strip electrodes is their ability to adequately support the contacts and lead wires secured by the dielectric strip. If insufficient dielectric supporting material encompasses the contacts and lead wires, the lead wires when pulled could distort the strip and create undesirable openings in the strip. The lead wires would also be more prone to break away and the contacts more prone to be mislocated within the strip.
All of these factors argue for greater width and thickness dimensions in the strip electrode--that is, greater cross-sectional area. Yet, there has been a trend in the subdural strip electrode art to decrease cross-sectional area.
A problem resulting from low mass and low cross-sectional area in subdural strip electrodes is that they are not easily seen under X-ray.
The lead wires are generally routed out through a stab wound in the skin remote from the electrode and generally terminate in a distal end with ring-type terminals (FIG. 4). These ring-type terminals are then connected to monitoring equipment by means of a connector. Such a prior connector is disclosed in U.S. Pat. No. 4,869,255. FIG. 7 illustrates such a connector of the prior art.
The connector 20 holds several pairs of male conductor members 34 in an array. The lead wire terminal rings 32 of the electrode and male conductor members 34 are held in engagement by mechanical interference. As can be seen in FIG. 7, this arrangement results in the lead wire terminal rings 32 making contact with the male conductor members 34 only along two short arcs. This minimal surface area may result in inaccurate electrical recordings of cortical activity.
There is a need for an improved tissue electrode and connector which addresses the above problems of prior art electrodes.